Hearing devices are designed to be used as medical aids to enable patients with hearing damage to hear as naturally as possible. In such cases care must be taken to suppress as completely as possible any interference noise caused by the technology involved. Such interference noise especially includes whistling noises caused by acoustic feedback. Acoustic feedback of this nature occurs especially with hearing device systems when said systems are operating with high amplification and are caused by oscillations at a specific frequency fed back to the microphone (feedback). In some cases whistling caused in this way is so loud that it is even perceived as disturbing in the vicinity of a hearing device wearer.
A whistling caused by feedback can occur whenever sound, which is picked up by a microphone of a hearing device, is amplified by a corresponding amplifier and output again via a sound-generating output unit, for example via the earpiece of a hearing device. In such cases the output sound might reach the microphone again and be further amplified. Two conditions must thus be fulfilled for feedback-induced whistling to occur. The sound amplification must be greater than the attenuation of the sound on the way from the sound-generating output unit back to the microphone. In addition the phase shift at the microphone between the sound originally picked up by the microphone and the sound sent out by the sound-generating output unit must correspond to 2Π or any given multiple thereof. There are numerous possible ways of countering the occurrence of feedback-induced whistling in hearing devices or hearing device systems by influencing these two conditions. One possibility consists of limiting the hearing device amplification, but, especially with a serious hearing impairment of the hearing aid wearer, this results in the function of the hearing device system overall being reduced ad absurdum.
Another known method is to reduce the loop amplification of a hearing device system or hearing device, that is the product of the hearing device amplification and the attenuation of the feedback path, during an adaptation of the hearing device by setting a so-called notch filter (narrowband blocking filter) in frequency ranges in which there is likely to be an occurrence of feedback. Since however especially the characteristic of the feedback path set is in some cases strongly dependent on the ambient conditions, the occurrence of acoustic feedback can in some cases not be safely avoided with such notch filters since their frequencies cannot be reliably predicted.
Methods are also known that are able, by a dynamic reduction of feedback oscillations, to adjust themselves automatically to different feedback situations and which are intended to take care of a corresponding suppression of these types of oscillation. So-called compensation algorithms are known, which with the aid of adaptive filters estimate the feedback component in a microphone signal and neutralize it by subtraction. In this way the hearing device amplification is not adversely affected and is available in its full capacity for the amplification of useful signals. A weakpoint of known compensation methods is the precision of the estimation of the proportion of the feedback signal. They are suitable for the separation of wideband input signals of feedback-induced oscillations. Tonal input signals however will however in some cases be interpreted as feedback-induced oscillation. As a result of an estimation of the feedback component in the microphone signal which is thus inevitably incorrect, the tonal input signal actually arriving as the useful signal can itself be subtracted.
The use of algorithms which become active after the detection of apparent feedback-induced oscillations is also known. In such cases the microphone signal is continuously monitored. After detection of an oscillation indicating feedback the amplification of the hearing device is reduced to a point at which the loop amplification falls below a critical limit. This reduction of the amplification can be undertaken by reducing the amplification within a specific frequency channel or can include the activation of a corresponding blocking filter. The disadvantage of such methods however is likewise that conventional oscillation detectors cannot distinguish between feedback-induced oscillations on the one hand and tonal narrowband input signals on the other hand. As a result tonal narrowband input signals can activate algorithms intended for the suppression of the feedback-induced oscillations and thereby themselves help to suppress their amplification.
A further known practice, especially in binaural hearing device systems, is to compare incoming microphone signals in order to contribute to distinguishing between feedback-induced oscillations and the useful signals that are in some cases similar to these oscillations (DE 10110258C1). This invention starts from the assumption that in binaural systems, on the one hand the amplification of the individual hearing device components will be set differently because of the adaptation to individual hearing damage, and on the other hand, by relatively small variations of the arrangement of the hearing device components at the ear of a wearer as well as by numerous ambient conditions in the vicinity of the hearing device wearer, different levels of attenuation of the individual feedback paths will be produced. For this reason it cannot be reckoned that spontaneously occurring feedback-induced oscillations will occur at both hearing device components at the same frequency. An incoming useful signal on the other hand will always be present almost simultaneously and with the same frequency at both components of a binaural hearing device system. By a comparison of the generated microphone signals using a so-called coherence analysis, an attempt is made to interpret signals with high coherence as useful signals and signals with low coherence as feedback-induced oscillations. A disadvantage of this method however lies in the fact that with a constant occurrence of feedback-induced oscillations at a component of a binaural hearing device system, this will be coupled-in after short time via the sound-generating output device into the microphone of the other component of the binaural hearing device system as well if the sound generated by the feedback-induced oscillations is emitted sufficiently loudly by the oscillating components. A coherence analysis inevitably produces a high level of coherence for these types of generated signals. This means that the signals will be interpreted as incoming useful signals. The misinterpretation results in no measures being undertaken to suppress the feedback-induced whistling.